This invention relates to a transmitter for use in CDMA (Code Division Multiple Access), which will be called a CDMA transmitter.
In a conventional CDMA method which will be known as a wide band CDMA method, such as TIA/EIA/IS-95, a base station has a CDMA transmitter which is operable in response to a plurality of input data signals. The CDMA transmitter spreads the input data signals by the use of short codes, such as Walsh codes, and long codes, such as pseudo random sequences, and thereafter transmits a transmission signal through a power amplifier downwards (namely, from the base station to mobile stations).
In the base station, the input data signals are individually subjected to spread spectrum modulation and are synthesized into a synthesized signal. The synthesized signal is varied in electric power over a very wide range, depending upon a peak value of each input data signal. Moreover, a linear amplifier is used as the power amplifier to amplify the synthesized signal into the transmission signal.
Under the circumstances, it should be noted that the power amplifier should have high power so as to linearly amplify the synthesized signal varied over the wide range and is therefore inevitably very expensive. Taking this into consideration, a limiter is arrange prior to the power amplifier to limit an amplitude of the synthesized signal. However, such arrangement of the limiter deteriorates a transmission characteristic of the CDMA transmitter because the limiter itself is a nonlinear element.
It is an object of this invention to provide a CDMA transmitter which is capable of reducing transmission power.
It is another object of this invention to provide a CDMA transmitter of the type described, which can establish a reduction of transmission power without degrading a transmission characteristic.
It is still another object of this invention to provide a CDMA transmitter of the type described, which is inexpensive and small in size.
A CDMA transmitter to which this invention is applicable is operable in response to a plurality of input data signals to transmit a transmission signal, by subjecting the input data signals to symbol modulation, spread spectrum modulation, synthesizing, and amplification. According to an aspect of this invention, the CDMA transmitter comprises a peak detector for detecting peak electric power of the input data signals synthesized to produce a peak power signal representative of the peak electric power, a dummy symbol generator, supplied with the peak power signal, for generating a dummy symbol such that the peak electric power is decreased, and a calculation circuit for calculating the synthesized input data signals and the dummy symbol to produce the transmission signal.
The peak detector is supplied with each symbol of the input data signals which are subjected to the symbol modulation and the spread spectrum modulation and which are added by an adder. The peak detector detects the peak electric power of each symbol.
In addition, the CDMA transmitter further may comprise a delay circuit which has a delay time delaying each synthesized input data signals by a single symbol time and which produces a delayed synthesized data signal. In this event, the calculation circuit comprises a subtractor for subtracting the dummy symbol from the delayed synthesized data signal.
Herein, the above-mentioned dummy symbol generator comprises a comparator for comparing the peak power signal with a predetermined power level to detect whether or not the peak power signal exceeds the predetermined power level and a generating circuit for generating the dummy symbol when the peak power signal exceeds the predetermined power level.
The dummy symbol generator generates the dummy symbol which is orthogonal to the synthesized input data signals and which is generated by the use of a code sequence which is unused in the synthesized input data signals. The dummy symbol is generated by the dummy symbol generator by the use of Walsh codes.
Specifically, the dummy symbol generator comprises a plurality of code generators for producing a plurality of codes different from one another, a weight calculator for calculating weight coefficients given to the codes, with reference to the peak power signal detected by the peak detector, a multiplier for multiplying the plurality of the codes by the weight coefficients to produce multiplied codes, and an adder circuit for summing up the multiplied codes to produce the dummy symbol.
Furthermore, a CDMA transmitter may comprise a power adjustment circuit for adjusting transmission electric power with reference to the peak power signal.
A CDMA transmitter according to another aspect of this invention is operable in response to a plurality of input data signals to produce a transmission signal and comprises a spread section for spectrum spreading the input data signals by using a plurality of predetermined spread codes to produce a synthesized signal, a dummy symbol generator for generating a dummy symbol which is subjected to spread spectrum modulation by the use of spread codes which are different from the predetermined spread codes, and a combining section for combining the synthesized signal with the dummy symbol.